Known monolithic ceramic electronic components such as monolithic ceramic capacitors have a configuration in which a plurality of plated layers are arranged on a sintered electrode layer. In a known technique disclosed in, for example, Patent Document 1 cited below, an intermediate electroplated layer made of, for example, Ni is disposed on a sintered electrode layer having a certain thickness and a plated layer made of Sn or a Sn alloy is disposed on the intermediate electroplated layer. This is because the plated layer made of Sn or a Sn alloy is enhanced in solderability and the intermediate electroplated layer is useful in preventing the oxidation and solder erosion of the sintered electrode layer.
It is known that various properties of a monolithic ceramic capacitor are deteriorated if a plating solution permeates the sintered electrode layer in a step of forming the intermediate electroplated layer and the outside plated Sn layer.
In high-capacity monolithic ceramic capacitors including a large number of thin ceramic layers disposed between internal electrodes in particular, there is a problem in that after plated Sn layers are formed, the reliability is reduced during a high temperature loading test. This is probably because the sintered electrode layers are not completely covered with the intermediate electroplated layers made of Ni. That is, a plating solution for forming the plated Sn layers probably flows through crevices in the plated Ni layers to dissolve an oxide, such as glass, contained in the sintered electrode layers to permeate the sintered electrode layers to reach ceramic bodies. This causes a decrease in insulation resistance or the like.
In order to enhance solderability by completely covering the sintered electrode layers, which are included in external electrodes, with the plated Ni layers, various attempts have been made. For example, Patent Document 2 cited below discloses that fine particles derived from a glass frit contained in a conductive paste for forming a sintered electrode layer are prevented from adhering to the sintered electrode layer in such a manner that the sintered electrode layer is subjected to barrel-polishing. Since the resulting sintered electrode layer therefore has high surface smoothness, the sintered electrode layer can be satisfactorily covered with an intermediate electroplated layer.
Furthermore, the following technique has been proposed: a glass frit contained in a conductive paste for forming a sintered electrode layer is replaced with a conductive glass frit. A technique in which a plated Ni layer, which is an electroplated layer, is increased in thickness has been also proposed.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-75779    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-117804
Even if barrel processing is performed as disclosed in Patent Document 2 such that the adhesion of the fine particles derived from the glass frit is prevented and the surface of the sintered electrode layer is smoothed, the insulation resistance is reduced due to the permeation of a plating solution for Sn plating during the deposition of an electroplated Ni layer and then a plated Sn layer on the sintered electrode layer. This is probably because even if the sintered electrode layer surface is smoothed by barrel polishing, bare oxides including the glass frit remain on the sintered electrode layer, the plating solution dissolves the oxides, pinholes are formed in surface portions of the sintered electrode layer from which the glass frit has been removed, and the plating solution flows through the pinholes during Sn plating.
In the technique in which the conductive glass frit is used instead of the above glass frit, although an electroplated Ni layer is deposited over regions exposed from the conductive glass frit, the thickness of the electroplated Ni layer is insufficient because the conductive glass frit is inferior in conductivity as compared to metal. Therefore, the insulation resistance is reduced due to the permeation of a plating solution for Sn plating when a high-temperature loading test is performed.
In the technique in which the electroplated Ni layer is increased in thickness, there is a problem in that the cost is high. Even if only the thickness of the electroplated Ni layer is increased, it is difficult to prevent a reduction in insulation resistance during the high-temperature loading test.